1. Field of the Invention
The present invention relates to an optical alignment polymer and an optical alignment composition having the same, and more particularly, to an optical alignment polymer having excellent optical alignment performance and high thermal stability.
2. Description of the Related Art
In general, a liquid crystal display device has upper and lower substrate, transparent electrodes and alignment layers formed on the upper and lower substrates and a liquid crystal layer between the upper and lower alignment layers.
In the LCD having the aforementioned structure, according to an externally applied voltage, the arrangement of liquid crystals is changed due to influence of the electric field. According to the changed arrangement, external light introduced to the LCD is shielded or transmitted. The LCD is driven by such a property. In other words, if a voltage is applied to the transparent electrode layers, an electric field is formed in the liquid crystal layer. Thus, liquid crystals are driven in a predetermined direction. The light introduced into the liquid crystals of the LCD is shielded or transmitted according to the driving of the liquid crystals.
The functional parameters of the LCD as a display device, i.e., light transmittance, response time, view angle or contrast, are determined by the arrangement characteristic of the liquid crystal molecules. Therefore, a technology of controlling the alignment of the liquid crystal molecules uniformly is a very important factor.
The uniform alignment state of the liquid crystals is difficult to accomplish by merely interposing the liquid crystals between the upper and lower substrates. Thus, it is conventional to form the alignment layers for aligning liquid crystals on the transparent electrode layers.
The alignment layer is conventionally formed by a rubbing method in which a thin film made of an organic polymer material such as polyimide or polyamide is formed, cured and then rubbed with a special cloth.
The rubbing method is easy to conduct and the process thereof is simple. However, minute particles or materials such as cellulose may separate from the cloth used in the rubbing treatment to contaminate the alignment layer. Further, depending on the material for forming the, alignment layer, the alignment may not be accomplished smoothly. Moreover, a thin film transistor may be damaged by static electricity generated during the rubbing treatment.
To solve the above-described problems, an optical alignment technology has been developed in which dust, static electricity or other pollutant particles are not generated and cleanliness is maintained during the overall process. According to such a non-destructive alignment method, polarized light is irradiated onto the optical alignment layer to cause anisotropic photopolymerization. As a result, the optical alignment layer has alignment characteristic, thereby uniformly aligning the liquid crystals.
As the polymer for the optical alignment layer, that is, the optical alignment polymer, a polymer having a cinnamate group such as poly(vinylcinnamate) (PVCN) or poly(vinylmethoxycinnamate) (PVMC), or a polymer having a coumarin group is widely used. These polymer compounds have good alignment performance by light, but have poor thermal stability in alignment.
To solve these problems, other kinds of optical alignment polymers formed from introduction of a photosensitive group such as cinnamate group introduced into a heat-resistant polymer, e.g., polyimide, having a high glass transition temperature (Tg) have been suggested. If the alignment layer is formed of such optical alignment polymers, the alignment performance by light and a pretilt angle of a liquid crystal of liquid crystal display device having the alignment layer improve. And, thermal stability is higher than that of conventional optical alignment polymers such as PVCN, but the above stability is not satisfactory. This is because the introduction density of the photosensitive groups is quite restricted due to characteristics of the repeating unit of the optical alignment polymers.
In a polyimide, carbon number per repeating unit is ten or more. Thus, it is difficult to introduce two or more photosensitive groups into the repeating unit of the polyimide due to structure characteristic of repeating unit. Thus, the introduction density of the photosensitive groups in the polyimide is low.